Andrew W. Knight

Understanding the Radioactive Ingrowth and Decay of Naturally Occurring Radioactive Materials in the Environment: An Analysis of Produced Fluids from the Marcellus Shale.

Background The economic value of unconventional natural gas resources has stimulated rapid globalization of horizontal drilling and hydraulic fracturing. However, natural radioactivity found in the large volumes of “produced fluids” generated by these technologies is emerging as an international environmental health concern. Current assessments of the radioactivity concentration in liquid wastes focus on a single element—radium. However, the use of radium alone to predict radioactivity concentrations can greatly underestimate total levels. Objective We investigated the contribution to radioactivity concentrations from naturally occurring radioactive materials (NORM), including uranium, thorium, actinium, radium, lead, bismuth, and polonium isotopes, to the total radioactivity of hydraulic fracturing wastes. Methods For this study we used established methods and developed new methods designed to quantitate NORM of public health concern that may be enriched in complex brines from hydraulic fracturing wastes. Specifically, we examined the use of high-purity germanium gamma spectrometry and isotope dilution alpha spectrometry to quantitate NORM. Results We observed that radium decay products were initially absent from produced fluids due to differences in solubility. However, in systems closed to the release of gaseous radon, our model predicted that decay products will begin to ingrow immediately and (under these closed-system conditions) can contribute to an increase in the total radioactivity for more than 100 years. Conclusions Accurate predictions of radioactivity concentrations are critical for estimating doses to potentially exposed individuals and the surrounding environment. These predictions must include an understanding of the geochemistry, decay properties, and ingrowth kinetics of radium and its decay product radionuclides. Citation Nelson AW, Eitrheim ES, Knight AW, May D, Mehrhoff MA, Shannon R, Litman R, Burnett WC, Forbes TZ, Schultz MK. 2015. Understanding the radioactive ingrowth and decay of naturally occurring radioactive materials in the environment: an analysis of produced fluids from the Marcellus Shale. Environ Health Perspect 123:689–696;u2002http://dx.doi.org/10.1289/ehp.1408855

A simple-rapid method to separate uranium, thorium, and protactinium for U-series age-dating of materials

Uranium-series dating techniques require the isolation of radionuclides in high yields and in fractions free of impurities. Within this context, we describe a novel-rapid method for the separation and purification of U, Th, and Pa. The method takes advantage of differences in the chemistry of U, Th, and Pa, utilizing a commercially-available extraction chromatographic resin (TEVA) and standard reagents. The elution behavior of U, Th, and Pa were optimized using liquid scintillation counting techniques and fractional purity was evaluated by alpha-spectrometry. The overall method was further assessed by isotope dilution alpha-spectrometry for the preliminary age determination of an ancient carbonate sample obtained from the Lake Bonneville site in western Utah (United States). Preliminary evaluations of the method produced elemental purity of greater than 99.99% and radiochemical recoveries exceeding 90% for U and Th and 85% for Pa. Excellent purity and yields (76% for U, 96% for Th and 55% for Pa) were also obtained for the analysis of the carbonate samples and the preliminary Pa and Th ages of about 39,000 years before present are consistent with (14)C-derived age of the material.

Monitoring radionuclides in subsurface drinking water sources near unconventional drilling operations: a pilot study.

Unconventional drilling (the combination of hydraulic fracturing and horizontal drilling) to extract oil and natural gas is expanding rapidly around the world. The rate of expansion challenges scientists and regulators to assess the risks of the new technologies on drinking water resources. One concern is the potential for subsurface drinking water resource contamination by naturally occurring radioactive materials co-extracted during unconventional drilling activities. Given the rate of expansion, opportunities to test drinking water resources in the pre- and post-fracturing setting are rare. This pilot study investigated the levels of natural uranium, lead-210, and polonium-210 in private drinking wells within 2000 m of a large-volume hydraulic fracturing operation--before and approximately one-year following the fracturing activities. Observed radionuclide concentrations in well waters tested did not exceed maximum contaminant levels recommended by state and federal agencies. No statistically-significant differences in radionuclide concentrations were observed in well-water samples collected before and after the hydraulic fracturing activities. Expanded monitoring of private drinking wells before and after hydraulic fracturing activities is needed to develop understanding of the potential for drinking water resource contamination from unconventional drilling and gas extraction activities.

A chromatographic separation of neptunium and protactinium using 1-octanol impregnated onto a solid phase support

We have developed a new chromatographic method to efficiently separate and isolate neptunium (Np) and protactinium (Pa), based on the selective extraction of protactinium by primary alcohols. The effectiveness of the new technology is demonstrated by efficient separation of 233Pa from parent radionuclide 237Np, using a hydrochloric acid mobile-phase medium. Our new approach reproducibly isolated 233Pa tracer with a yield of 99xa0±xa01xa0% (nxa0=xa03; radiochemical purity 100xa0%) and enabled chemical recovery of 237Np parent material of 92xa0±xa03xa0% (radiochemical >99xa0%) for future 233Pa tracer preparations. Compared to previous methods, the new approach reduces radioactive inorganic and organic waste; simplifies the separation process by eliminating cumbersome liquid–liquid extractions; and allows isolation of radiochemically-pure fractions in less than 1xa0h.Graphical Abstract

Subtle Effects of Aliphatic Alcohol Structure on Water Extraction and Solute Aggregation in Biphasic Water/n-Dodecane

Organic phase aggregation behavior of 1-octanol and its structural isomer, 2-ethylhexanol, in a biphasic n-dodecane-water system is studied with a combination of physical measurement, small-angle X-ray scattering (SAXS), and atomistic molecular dynamic simulations. Physical properties of the organic phases are probed following their mixing and equilibration with immiscible water phases. Studies reveal that the interfacial tension decreases as a function of increasing alcohol concentration over the solubility range of the alcohol with no evidence for a critical aggregate concentration (cac). An uptake of water into the organic phases is quantified, as a function of alcohol content, by Karl Fischer titrations. The extraction of water into dodecane was further assessed as a function of alcohol concentration via the slope-analysis method sometimes employed in chemical separations. This method provides a qualitative understanding of solute (water/alcohol) aggregation in the organic phase. The physical results are supported by analyses of SAXS data that reveals an emergence of aggregates in n-dodecane at elevated alcohol concentrations. The observed aggregate structure is dependent on the alcohol tail group geometry, consistent with surfactant packing parameter. The formation of these aggregates is discussed at a molecular level, where alcohol-alcohol and alcohol-water H-bonding interactions likely dominate the occurrence and morphology of the aggregates.

A calculation model for liquid-liquid extraction of protactinium by 2,6-dimethyl-4-heptanol

Abstract Reprocessing of spent nuclear fuel usually employs the solvent extraction technique to recover fissile material, isolate other valuable radionuclides, recover precious metals, and remove contaminants. Efficient recovery of these species from highly radioactive solutions requires a detailed understanding of reaction conditions and metal speciation that leads to their isolation in pure forms. Due to the complex nature of these systems, identification of ideal reaction conditions for the efficient extraction of specific metals can be challenging. Thus, the development of experimental approaches that have the potential to reduce the number of experiments required to identify ideal conditions are desirable. In this study, a full-factorial experimental design was used to identify the main effects and variable interactions of three chemical parameters on the extraction of protactinium (Pa). Specifically we investigated the main effects of the anion concentration (NO3-, Cl-) extractant concentration, and solution acidity on the overall extraction of protactinium by 2,6-dimethyl-4-heptanol (diisobutylcarbinol; DIBC) from both HCl and HNO3 solutions. Our results indicate that in HCl, the extraction of protactinium was dominated by the solution acidity, while in nitric acid the extraction was strongly effected by the [DIBC]. Based on our results, a mathematical model was derived, that describes the relationship between concentrations of anions, extractant, and solution acidity and the expected values of Pa distribution coefficients in both HCl and HNO3. This study demonstrates the potential to predict the distribution coefficient values, based upon a mathematical model generated by a full-factorial experimental design.

Separation of gallium and actinides in plutonium nuclear materials by extraction chromatography

Analysis of stable gallium in nuclear materials has applications in nuclear fuel characterization and nuclear forensics. The use of positron-emitting gallium isotope 68Ga as a tracer for Ga recoveries for analyses in materials containing actinides was explored. A radiochemical method for the separation of Ga, Pu, U, Th, and Am using commercially-available extraction chromatography resins was developed and evaluated. The method effectively allows precise determination of Ga yield (97xa0±xa03xa0%) in the analysis of stable Ga (spike recovery 101xa0±xa01xa0%) and radioactive Pu (radiochemical yield, 82xa0±xa010xa0%; spike recovery, 96xa0±xa03xa0%), while also providing pure elemental fractions of other actinides relevant to materials encountered in the analysis Pu-containing materials.

Extraction Selectivity of a Quaternary Alkylammonium Salt for Trivalent Actinides over Trivalent Lanthanides: Does Extractant Aggregation Play a Role?

ABSTRACT The extraction behavior of a quaternary alkylammonium salt extractant was investigated for its selectivity for trivalent actinides over trivalent lanthanides in nitrate and thiocyanate media. The selectivity was evaluated by solvent extraction experiments through radiochemical analysis of 241Am and 152/154Eu. Solvent extraction distribution and slope-analysis experiments were performed with americium(III) and europium(III) with respect to the ligand (nitrate and thiocyanate), extractant, and metal (europium only) concentrations. Further evaluation of the equilibrium expression that governs the extraction process indicated the appropriate use of the saturation method for estimation of the aggregation state of quaternary ammonium extractants in the organic phase. From the saturation method, we observed an average aggregation number of 5.4 ± 0.8 and 8.5 ± 0.9 monomers/aggregate for nitrate and thiocyanate, respectively. Through a side-by-side comparison of the nitrate and thiocyanate forms, we discuss the potential role of the aggregation in the increased selectivity for trivalent actinides over trivalent lanthanides in thiocyanate media.

Polonium-210 accumulates in a lake receiving coal mine discharges-anthropogenic or natural?

Coal is an integral part of global energy production; however, coal mining is associated with numerous environmental health impacts. It is well documented that coal-mine waste can contaminate the environment with naturally-occurring radionuclides from the uranium-238 (238U) decay series. However, the behavior of the final radionuclide in the 238U-series, i.e., polonium-210 (210Po) arising from coal-mine waste-water discharge is largely unexplored. Here, results of a year-long (2014-2015) field study, in which the concentrations of 210Po in sediments and surface water of a lake that receives coal-mine waste-water discharge in West Virginia are presented. Initial measurements identified levels of 210Po in the lake sediments that were in excess of that which could be attributed to ambient U-series parent radionuclides; and were indicative of discharge site contamination of the lake ecosystem. However, control sediment obtained from a similar lake system in Iowa (an area with no coal mining or unconventional drilling) suggests that the levels of 210Po in the lake are a natural phenomenon; and are likely unrelated to waste-water treatment discharges. Elevated levels of 210Po have been reported in lake bottom sediments previously, yet very little information is available on the radioecological implications of 210Po accumulation in lake bottom sediments. The findings of this study suggest that (Monthly Energy Review, 2016) the natural accumulation and retention of 210Po in lake sediments may be a greater than previously considered (Chadwick etxa0al., 2013) careful selection of control sites is important to prevent the inappropriate attribution of elevated levels of NORM in lake bottom ecosystems to industrial sources; and (Van Hook, 1979) further investigation of the source-terms and potential impacts on elevated 210Po in lake-sediment ecosystems is warranted.

Trace-Level Extraction Behavior of Actinide Elements by Aliphatic Alcohol Extractants in Mineral Acids: Insights into the Trace Solution Chemistry of Protactinium

ABSTRACTThe extraction of actinide elements thorium, protactinium (Pa), uranium, neptunium, and americium by aliphatic alcohols (1-octanol, 2-ethyl-hexanol, and 2,6-dimethyl-4-heptanol) was investigated with solvent extraction and extraction chromatographic techniques from hydrochloric and nitric acid solutions. These systems provide the potential for low-cost, high quality methods for the isolation of Pa from complex matrices. Acid dependency experiments demonstrate the selective extraction of protactinium from hydrochloric and nitric acids, relative to the other actinides explored. Experiments were conducted to elucidate the equilibrium chemical stoichiometry of the protactinium complex that underlies this unique extraction behavior. Slope analysis with respect to the alcohol concentration infers a stoichiometric relationship of 2:1 for the alcohol extractant to the protactinium ion. Slope analysis with respect to the chloride ion (when [H+] = 0.1, 1, and 4 M) suggests that the stoichiometric identity o...ABSTRACT The extraction of actinide elements thorium, protactinium (Pa), uranium, neptunium, and americium by aliphatic alcohols (1-octanol, 2-ethyl-hexanol, and 2,6-dimethyl-4-heptanol) was investigated with solvent extraction and extraction chromatographic techniques from hydrochloric and nitric acid solutions. These systems provide the potential for low-cost, high quality methods for the isolation of Pa from complex matrices. Acid dependency experiments demonstrate the selective extraction of protactinium from hydrochloric and nitric acids, relative to the other actinides explored. Experiments were conducted to elucidate the equilibrium chemical stoichiometry of the protactinium complex that underlies this unique extraction behavior. Slope analysis with respect to the alcohol concentration infers a stoichiometric relationship of 2:1 for the alcohol extractant to the protactinium ion. Slope analysis with respect to the chloride ion (when [H+] = 0.1, 1, and 4 M) suggests that the stoichiometric identity of the protactinium chloro-complexes depends on the [H+] (0.1 M to 4 M). Extraction of Pa increases as the Pa:Cl ratio increases from 1:2, at low acid concentration, to 1:6 at high acid concentration. With respect to the nitrate ion (when [H+] = 1 and 4 M), the stoichiometric relationship was determined to be 2:1 nitrate to protactinium throughout the range investigated. The sum of these findings and observations contribute to a deeper understanding of the unique chemistry of protactinium compared to the other members of the actinide group.